1. Field of the Invention
This invention relates to an apparatus for measuring variations in flatness of an Almen strip holding surface used in quality control of shot peening operations.
2. Description of the Background of the Invention
Shot peening, blast cleaning, and similar operations are well known and are widely practiced. However, the intensity of shot peening or blast cleaning must be carefully controlled, as either too great or insufficient shot peening or blasting can be detrimental. Accordingly, a process using an “Almen strip” has been developed in which a flat metal strip having standardized dimension is secured to a flat holding surface, and then either peened or blasted for a predetermined time. Accordingly, residual stresses are introduced into the Almen strip, and the Almen strip relaxes into a natural curve when removed from the holding surface. The degree of curvature is an indication of the intensity of the peening or blasting. Well known devices are widely available for reading this curvature, thus providing a record of the intensity of the subsequent peening process. In order that such readings be reproducible, it is necessary to assure that the surface upon which the Almen strip is supported during peening is absolutely flat (within a predetermined tolerance). Any variation of the surface, even along the edges, can result in corrupted readings. The edges of the holding surface are particularly important, since they are more likely to be damaged by repeated peening into a non-flat state. Another factor influencing the readings of Almen strips is variation in the location of the hold down screws which secure the Almen strip to the holding surface.
U.S. Pat. No. 6,148,532 discloses a gage measuring flatness of an Almen strip holding surface. However, the apparatus disclosed in this patent permits measurement only along the center line of the holding surface and cannot measure flatness along the edges. Also, there is no way to confirm that the apertures receiving the Almen strip hold down screws are within tolerance.